Plastic packages such as video cases have been manufactured for many years. The typical process consists of thermoforming a relatively rigid thermoplastic sheet, such as polyvinyl chloride (PVC), to form opposed inner contoured surfaces configured to receive and hold the video cassette or other object when the case is closed by folding about its spine, then covering the back side of the rigid sheet with a cardboard stiffener, an opaque relatively flexible PVC sheet, and a clear relatively flexible PVC sheet, followed by heat-sealing together the sandwich so formed so as to seal the cardboard stiffener between the rigid PVC and the flexible opaque PVC sheets and the latter to each other and to the clear PVC sheet around three sides so that advertising material for the contents of the case can be inserted through the unsealed fourth side and thus be visible to prospective customers and users of the video cassette. Typically, for PVC, the relatively rigid sheet has a thickness between about 0.008 and 0.030 inches (in.), and the relatively flexible sheets have thicknesses ranging between about 0.003 and 0.020 in. The term "rigid" or "relatively rigid" is a term of art meaning a sheet thickness that will hold its shape when thermoformed, is usually but not always thicker than the flexible sheet or relatively flexible sheets, but still has sufficient flexibility to allow the case to bend easily around its spine and thus easily open and close. However, this degree of flexibility is insufficient to stiffen the case sides, which in use are not supposed to bend. Hence, it is common practice to insert a stiffener member in the case sides to stiffen the latter. Typically, the stiffener member is made of a non-heat-sealable material such as, for example, inexpensive cardboard about 0.018 to 0.120 in. thick, with score lines or slits defining a center spine to allow the flat sides of the stiffener member to bend around the score lines or slits when the case is closed.
The trend for many years has been to reduce the manufacturing cost of such cases. The major expense is labor, and a major way to reduce labor costs is to increase production. Thus, it is common to use so-called turntable machines in the sandwich assembly process, during which at stations situated around the turntable the case elements, including the thermoformed rigid member, the stiffener member, and the two flexible sheets, are assembled and at a final station the assembled sandwiched elements are heat-sealed together as described above.
By rotating the turntable faster, production rate can be increased, but I have found that .the main bottleneck to increasing turntable speed is not the heat-sealing time nor the ability of the production personnel to keep up with the rotating table, but rather the ability to maintain the stiffener member in its proper position aligned with the rigid and flexible members when subjected to the increased centrifugal forces while the turntable is rotating at its faster speed from station to station before the assembly is heat-sealed. Various tricks have been used to fix the position of the stiffener member in the pre-sealed assembly, such as by using pins or adhesives to hold the stiffener member to the rigid member, but these measures have introduced additional steps in the manufacturing process with the result that only minor increases in production rates have resulted.